1. Field of the Invention
The invention relates to variable camber vanes as used in turbomachines, especially aircraft turbo-engines.
2. Summary of the Prior Art
To improve the performance of a turbomachine which can operate at different speeds, it is known to modify the pitch of certain vanes depending on the turbomachine speed.
A vane comprises a leading edge and a trailing edge joined together by two flanks termed the intrados face and the extrados face. The intrados face is generally concave and is inclined towards the upstream region of the fluid flow, while the extrados face is convex and is inclined towards the downstream region of the fluid flow. The fluid exerts a greater pressure on the intrados face than it does on the extrados face. The term "mean surface" is applied to the notional surface extending from the leading edge to the trailing edge mid-way between the intrados face and the extrados face. In more advanced machines, the camber of certain vanes may also be varied depending on the turbomachine speed. This type of vane usually has a front part and a rear part which are mutually pivotally connected, and the junction between the front part and the rear part provides both for the continuity of the intrados face and the extrados face, as well for the sealing between these two faces. The junction is defined by a concave surface on one of the parts and having the shape of a portion of a surface of revolution about the geometric axis of the pivotal connection, and a convex surface on the other part positioned closely to the concave surface, one of these two surfaces being movable angularly relative to the other around the pivot axis. This axis is generally situated in close proximity to the mean surface.
The closeness of the concave and convex surfaces of the junction ensures the seal between the intrados face and the extrados face. When the camber of the vane is increased, the convex surface of the junction is no longer fully covered by the concave surface, and contributes to the continuity of the extrados face.
Variable camber vanes are known having a fixed leading edge and a movable trailing edge, European Patent 0274293 disclosing an example of such a vane in the form of an air intake guide vane in which the leading edge also forms a radial arm providing a connection between the air intake casing of an aircraft turbo-engine and its hub.
Variable camber vanes are also known in which the leading edge pivots and the trailing edge is fixed, such as described in French patent 2325831.
Furthermore, variable camber vanes are known in which the leading and trailing edges both pivot, such as disclosed in U.S. Pat. No. 4,995,786.
In these three types of vane, the improvement in performance obtained by setting the camber of the vane in the best way possible is impaired by the disturbances caused in the flow of fluid along the lateral faces of the vane and in the region of the pivotal connection. These disturbances are caused both by the space left at the level of the pivotal connection between the leading edge part and the trailing edge part, and by the discontinuity of the extrados face of the vane at the level of the pivotal connection, that is to say by the sudden change of direction of the tangent to this face in the direction of fluid flow.
This sudden change of direction is particularly troublesome on the extrados face, as it produces a very localized overspeed at the level of the pivotal connection, as well as random separations of the fluid flow which create abnormal mechanical stresses on the vanes and losses in the performance of the turbomachine.
German Patent 42.12880 proposes a hinged vane in which the thickness of the front part is considerably increased relative to that of the rear part, the front part thus surrounding the pivotal connection much more. This solution improves the continuity of the extrados face when the camber of the vane is adjusted to the maximum, but the performance of the vane is impaired in all other configurations. This is due to the over-thickness of the front part of the vane, as well as to the simultaneous break in the continuity of the intrados face and the extrados face at the pivotal connection.
Generally therefore, the improvement in performance of a turbomachine made possible over wider operational ranges by the possibility of varying the camber of certain vanes is partly lost as a result of the impairment of the aerodynamic performance of the vane.